1. Field of the Invention
This invention relates generally to bidirectional serial data buses, and more specifically to bidirectional repeating, switching, and multiplexing of data signals on a serial bus without the need for passgates.
2. Description of the Related Art
A commonly used bidirectional serial data bus used for inter-integrated circuit communication is known as 12C. Each device transmitting and/or receiving data to or from the bus has an input/output (I/O) terminal coupled to a line of the data bus. Within a first such device, the I/O terminal is coupled to the drain or collector of an active pull-down transistor (hereinafter referred to as active pull-down) having its source or emitter grounded and a comparator. The comparator has a threshold voltage typically midway between logic “low” and logic “high” voltages so as to differentiate between a received low state and high state. Data to be transmitted to another device is coupled to the gate or base of the active pull-down in the first device, such that a logic “low” turns on the active pull-down causing a low voltage on the bus. During a logic “high”, the active pull-down is off, and the passive pull-up on the bus causes a high voltage on the bus. When the first device is receiving data, its active pull-down is off, and data received from the other device is compared in the first device to the threshold voltage, and appears as a logic “high” or “low” at the output of the comparator. The passive pull-up on the bus provides a logic “high” when the active pull-downs of all devices on the bus are off.
Each device and each bus line also has parasitic capacitance, shown for convenience in figures discussed in this document as capacitors to ground at each I/O node, but in fact distributed throughout the bus structure. The high-to-low transition speed of data on the bus is primarily affected by the on resistance of the active pull-down and this parasitic capacitance, while the low-to-high transition speed is primarily affected by the passive pull-up resistance and the parasitic capacitance. The cumulative parasitic capacitance on the bus increases as more devices are added to the bus and as the bus length increases, further slowing transitions. Similarly, the output data low level Vol is a function of the on resistance of the active pull-down at the transmit device and the combined resistance of the passive pull-ups on the bus.
When it is desired to couple data to/from two or more devices to a next device, often referred to as multiplexing the data signals, switched passgates are typically used to control which of the plurality of devices has access to the bus at a given time. These passgates are often implemented with metal oxide semiconductor field effect transistors (MOSFETs) having source and drain coupled in series with each signal line to be multiplexed. The passgate transistor in the signal line which has access to the bus is turned on, providing a low-resistance path for the data, while the passgates on the other lines remain off.
The non-zero on resistance of the passgate, however, further slows the rise and fall times of the data transitions, due to the parasitic capacitance at the I/O terminals of each device and parasitic capacitance of the bus itself. The voltage drop across this on resistance also lowers the input high voltage Vih and raises the input low voltage Vil appearing at the next device, thus decreasing the noise margin by reducing the peak to peak data voltage swing. Passgates also typically cannot isolate capacitance from one section of the bus to another.
It is desirable therefore to have a bidirectional repeater which isolates the parasitic capacitance between sections of the bus in a non-multiplexed or passgate multiplexed system. It is also desirable to have adjustability of some parameters of such a bidirectional repeater, to optimize its operation dependent on what type of logic it is coupled to. Further, it is desirable to have an alternate multiplexing mechanism eliminating passgates, and having capacitance isolation from device to device and reduced voltage drop across the multiplexing element, for improved noise margin.